Process control systems require the accurate measurement of process variables. Typically, a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal proportional to sensed level.
An electrical transmitter must be connected to an electrical power source to operate. One form of such a transmitter, known as a four-wire transmitter, includes two terminals for connection to a power source and two terminals for carrying a loop signal proportional to the process variable. This signal can be used as an input to a controller or for purposes of indication. Because the instrument is connected directly to a power source, power consumption is a less critical factor in design and use of the same.
The use of a four-wire transmitter, as discussed above, requires the use of four conductors between the transmitter and related loop control and power components. Where transmitters are remotely located, such a requirement is undesirable owing to the significant cost of cabling. To avoid this problem, instrument manufactures have strived to develop devices known as two-wire, or loop powered, transmitters. A two-wire transmitter includes two terminals connecting to a remote power source, with the transmitter controlling loop current drawn from the power source proportional to the process variable. A typical instrument operates off of a 24 volt DC source and varies the signal current in the loop between four and twenty milliamps DC. Because of these operating requirements the design of the transmitter in terms of power consumption is critical. For example, when a low level signal of four milliamps is transmitted, there is minimal power available to be consumed by the instrument. Therefore, circuits must be designed to operate off of such minimal available power.
One known form of level sensing instruments is an ultrasonic through-air, or sonar, transmitter. Such a transmitter transmits pulses of sound energy through the air above a liquid surface. Since the sound energy is at a frequency above that audible to humans, it is called ultrasound. The pulse of sound energy is provided by a transducer which acts to change electrical energy to mechanical vibration. The pulse of sound travels down from the transducer face to the liquid surface and is reflected back to the transducer. The transducer converts mechanical vibration from the received sound pulse back into an electrical signal. The transmitter circuitry monitors the time of flight from the transmission to the receipt of the sound pulse to determine the fluid distance from the transducer, i.e. level.
A typical transducer, such as a piezoelectric crystal, as well as the receiver and amplification circuits, have high power requirements. This requirement is compounded with a through air transmitter, as opposed to a through liquid transmitter, as air is a vibration insulator, and in process control applications in which greater level spans, such as on the order of thirty feet, are necessary. Thus, ultrasonic transmitters have generally been of the four-wire type.
Further, it is desirable that a level transmitter be operated in hazardous locations in which the transmitter cannot cause the ignition of hazardous gases or liquids. While to some extent this problem can be addressed using explosion-proof housing, it is desirable to use circuits with limited energy storage to prevent spark occurrence.
The present invention is directed to overcoming one or more of the problems set forth above.